1. Field of the Invention
This invention relates to a technology for printing images on a print medium using a bi-directional reciprocating movement in a main scanning direction. The invention particularly relates to a technology for correcting printing positional deviation between forward and reverse passes.
2. Description of the Related Art
In recent years color printers that emit colored inks from a print head are coming into widespread use as computer output devices. In recent years, such color printers have been devised as multilevel printers able to print each pixel using a plurality of dots having different sizes. Such printers use relatively small ink droplets to form relatively small dots on a pixel position, and relatively large ink droplets to form relatively large dots on a pixel position. These printers can also print bi-directionally to increase the printing speed.
A problem that readily arises in bi-directional printing is that of deviation in printing position between forward and reverse printing passes in the main scanning direction caused by backlash in the main scanning drive mechanism and warping of the platen that supports the print media. JP-A-5-69625 is an example of a technology disclosed by the present applicants for solving this problem of positional deviation. This comprises of registering beforehand the printing deviation amount in the main scanning direction and using this printing deviation amount as a basis for correcting the positions at which dots are printed during forward and reverse passes.
However, in the case of bi-directional printing using multilevel printers, little consideration has been given to positional deviation arising between forward and reverse printing passes. Other problems include that while deviation may be corrected with respect to a particular one of the multiple colored inks, there is no correction of deviation in other ink colors. As a result, the deviation correction provides little improvement in the quality of the color image. The effect that positional deviation has on image quality is particularly large in halftone regions.
An object of the present invention is to improve image quality by alleviating printing positional deviation arising between forward and reverse passes in the main scanning direction during bi-directional printing.
In order to attain at least part of the above and other objects of the present invention, a reference correction value is set for correcting printing positional deviation arising between forward and reverse main scanning passes with respect to specific reference dots. An adjustment value is determined, using at least the reference correction value, to reduce printing positional deviation arising between forward and reverse main scanning passes. The printing positional deviation between forward and reverse main scanning passes is adjusted using the adjustment value. In a first adjustment mode, the adjustment value is determined by correcting the reference correction value with a relative correction value prepared beforehand for correcting the reference correction value.
This arrangement improves image quality under various printing conditions by alleviating printing positional deviation arising between forward and reverse passes in the main scanning direction.
When the print head has a plurality of nozzle rows, the reference correction value may be a correction value for correcting printing positional deviation arising between forward and reverse main scanning passes with respect to a reference row of nozzles, and the relative correction value may be a correction value for correcting relative printing positional deviation of another row against the reference row. This arrangement reduces printing positional deviation relating to another row of nozzles other than the reference row of nozzles.
The reference row may a row of nozzles for emitting black ink and the another row may include a row of nozzles for emitting chromatic color ink.
The relative correction value may be applied in common to the rows of nozzles other than the reference row.
Alternatively, the relative correction values may be applied independently to respective rows of nozzles other than the reference row. This arrangement effectively reduces printing positional deviation of each row of nozzles.
The relative correction values may be applied independently to respective groups of nozzles for emitting respective inks. The amount of relative printing positional deviation depends on the properties of the ink, so printing positional deviation can be more effectively reduced by applying relative correction values on an individual, ink-by-ink basis.
When the print head is capable of printing N types (where N is an integer of 2 or more) of dots which are different at least in size, the reference dots may be one type of dots selected from among the N types of dots. In this case, the adjustment value may be applied in common to the N types of dots in the first adjustment mode. In this way, the printing positional deviation can be alleviated with respect to N types of dots, improving image quality.
The reference dots are preferably largest of the N types of dots. Thus, when a test pattern for setting the reference correction value is printed using the largest dots, it is easy to detect positional deviation on the pattern, thereby facilitating the setting of the reference correction values.
The relative correction value may substantially represent a difference between an amount of positional deviation relating to target dots and an amount of positional deviation relating to the reference dots, where the target dots include at least one type of dots among the N types of dots, and where the at least one type of dots include dots smaller than the reference dots. This arrangement reduces positional deviation of the target dots that affect image quality.
The target dots may be smallest of the N types of dots. In many cases, image degradation tends to be more noticeable in places where the image density is relatively low, and the smallest size dots are used extensively when the image density is relatively low. As such, image quality in low-density regions can be improved by selecting the smallest dots to use as the target dots for reducing positional deviation.
The target dots may include plural types of dots of different sizes, and an average of the positional deviation amounts of the plural types of dots may be used as the positional deviation amount for the target dots. This arrangement reduces printing positional deviation with respect to plural types of dots that have a relatively large influence on image quality.
The reference dots may be formed of black ink and the target dots may be formed of chromatic color ink. Using black dots to print a test pattern for determining the reference correction value makes it easier to perceive deviations on the pattern, thereby facilitating the setting of the reference correction value. In the case of color images, dots printed in chromatic color inks affect the image quality to a major degree, so the quality of color images can be improved by reducing positional deviation of chromatic color ink dots.
The adjustment value may be determined in a second adjustment mode in which the reference correction value is used as the adjustment value. This adjustment value is used to adjust positional deviation of at least the reference dots. When positional deviation of reference dots is particularly noticeable, this reduces such deviation.
The printing positional deviation may be adjusted in accordance with the first adjustment mode during color printing, and in accordance with the second adjustment mode during monochrome printing. When printing in color, the overall positional deviation of the rows of nozzles is reduced, while during monochrome printing the positional deviation of just the reference row of nozzles (black-ink nozzles, in this case) is reduced. Thus, printing positional deviation can be effectively reduced when printing in color and when printing in monochrome.
The reference correction value may be determined according to correction information indicative of a preferred correction state that is selected from among test patterns of positional deviation printed using the reference dots. This facilitates the setting of the reference correction value.
When the bi-directional printing apparatus is capable of performing main scanning at a plurality of main scanning velocities, the relative correction values may be applied independently to the plurality of main scanning velocities. Since the relative degree of printing positional deviation depends on the main scanning velocity, such deviation can be effectively reduced by applying individual relative correction values for each main scanning velocity.
When the bi-directional printing apparatus is capable of emitting ink in a plurality of dot emission modes of mutually different ink emission velocities, the relative correction values may be applied independently to the plurality of dot emission modes. Since the relative degree of printing positional deviation depends on the ink emission velocity, such deviation can be effectively reduced by applying individual relative correction values for each ink emission velocity.
The second memory is preferably a non-volatile memory provided within the bi-directional printing apparatus.
Furthermore, the second memory is preferably attached to the print head so that the print head with the second memory is detachably attached to the bi-directional printing apparatus. Thus, when a print head is replaced, the relative correction value specifically for the new print head is used to reduce the printing positional deviation.
Specific aspects of the invention can be applied to various types of printing apparatus, printing methods, computer programs for implementing the printing apparatus or printing methods, computer program products storing the computer programs, and data signals embodied in a carrier wave including the computer programs.